A cellular telephone is a two radio transceiver communicating with a network of base stations as the mobile subscriber travels through the service area. The base stations are connected to a mobile telephone switching office (hereinafter “MSO”). The MSO links the cellular network to the land based public switch telephone network (hereinafter “PSTN”). A series of signals passes between a wireless mobile cellular device and a mobile base station. These signals enable the wireless mobile device or telephone to operate in a similar fashion as a wired land line phone.
The cellular system uses a finite number of frequencies that are shared amongst the multiple wireless telephone devices communicating through the cellular system. As such, it is possible that a mobile unit may receive signals from more than one base-station at one time. It is also possible that as a mobile unit moves through a coverage area, the signal strength of the transmitted or received signal may fall below a useable level. Other fading and interference effects may also prevent a clear connection. Further, the mobile may implement a scheme called discontinuous transmission (hereinafter “DTx”) in which speech is only transmitted when it is present. In order to handle these problems, the cellular base-station transmits “color code” symbols. Color code symbols are dedicated digital symbols in the transmission assigned to cells within the system. When a mobile device receives and decodes a signal, the mobile device then checks that the color code symbol received matches the color code for the service area cell to which the mobile device is currently assigned. If the color code does not match, the speech is discarded and not played out. Color code matching prevents cross talk in a speech conversation. Color code matching is done from the base-station to the mobile device as well as in a reverse direction. During a conversation, both the base-station and the mobile device continuously send the assigned color codes to each other, embedded in the transmission
In order to place a call, a user enters the desired number and presses send on their mobile device. The phone quickly measures the signal strength on the active access channels, and tunes to the strongest available signal strength. The mobile device then transmits identifying information and the number to call to the base-station. The base-station forwards this same information to the MSO. The base-station sets up the channel, using the appropriate color code for the cell, and begins sending the assigned color code embedded in the transmission to the mobile device. The MSO also out-pulses the call number to the PSTN, if the device is calling a land line telephone; or sends a paging message if the device is calling another mobile device. The mobile device receives and decodes the assigned voice channel and verifies that the color code is correct. If the color code is correct, the mobile device decodes the audio to the handset speaker. The base-station detects a reverse color code that the mobile device is sending. The base-station also un-mutes the reverse audio provided that the color code is correct. The checking of the color code is done on a frame-by-frame basis in both directions, independently.
The color codes not only assist in reducing interference, they also allow frequency reuse across sites and cellular networks. The frequency reused in geographically separated cells ensures radio spectrum efficiency but introduces undesirable co-channel interference that may result in dropped calls, hand off failure or cross talk in cases when the color codes are misinterpreted due to noise, resulting in a symbol error. What is needed is a method and a system that can be deployed to eliminate or reduce cross channel interference resulting in dropped calls, hand off failure or cross talk experienced by a mobile device user.